1. Field of the Invention
The present invention relates to a device for determining a hemodynamic parameter of a patient by means of pulse contour analysis.
2. The Prior Art
The determination of hemodynamic parameters, particularly the heart/time volume (cardiac output CO), by means of pulse contour analysis on the basis of a non-linear air dome model, has been described in detail in German Patent No. DE 198 14 371 A1 as well as in the literature listed there, which goes even further. The fundamental measurement variable for pulse contour analysis is a pressure that approximately corresponds to the aorta pressure, which is continuously measured, for example, by means of an arterial catheter in a leg artery. A pulse contour analysis system from Pulsion Medical Systems AG is commercially available under the designation PiCCO.
Significant variables in the determination of hemodynamic parameters, proceeding from the function P(t), i.e. the time progression of the pressure signal that approximately corresponds to the aorta pressure, are, in particular, the systemic vascular resistance (SVR), as well as furthermore the so-called compliance (C). The former is explained and understood as the flow-through resistance of the vascular system of the large blood circulation system, the latter as the resilience in the region of the aorta. In a substitute schematic, these variables can be represented as resistance and capacitance. In the case of older approaches, in particular, the compliance is sometimes ignored.
A device and a method for determining the compliance are disclosed in DE 198 14 371 A1.
In the case of conventional implementations of pulse contour analysis, calibration values that are determined within the scope of a calibration measurement and not changed after that are included in the determination of the systemic vascular resistance and the compliance (unless the latter is ignored). This calibration measurement includes the determination of a calibration value of the heart/time volume by trans-pulmonary thermo-dilution measurement.
For the heart/time volume determined by pulse contour analysis (pulse contour cardiac output, PCCO), which is calculated as the product of pulse frequency (heart rate, HR) and stroke volume (SV), the following equations are used.
The stroke volume (SV) is calculated by integration over a pulse period, or over the systole, according to the equation
  SV  ∝      ∫                  (                                            p              ⁡                              (                t                )                                      SVR                    +                                    C              ⁡                              (                p                )                                      ·                                          ⅆ                p                                            ⅆ                t                                                    )            ⁢              ⅆ        t            (with time t, pressure p, systemic vascular resistance SVR, compliance C).
In this connection, k is evaluated by numerical integration for every pulse period k, whereby the compliance is inserted in the form
      C    ⁡          (      p      )        =                    CO        TD                              〈                                    ⅆ              p                                      ⅆ              t                                〉                Cal              ·          1                                    3                          MAP              Cal                                ⁢          p                -        3        -                              1                          MAP              Cal              2                                ⁢                      p            2                              (with COTD:=calibration value of the heart/time volume determined by means of trans-pulmonary thermo-dilution measurement, <dp/dt>Cal:=mean [negative] incline of the pressure curve in the diastole during the calibration measurement, Pd:=diastolic pressure; MAPCal=mean arterial pressure during the calibration measurement).
The parameters COTD, MAPCal and <dp/dt>Cal are only determined within the scope of the calibration measurement and then used for all the heart/time volume calculations.
For the systemic vascular resistance SVR, as well, a calibration value determined in accordance with the equationSVRCal=(MAP−CVP)Cal/COTDis used (with MAP:=mean arterial pressure, CVP:=central venous pressure), which value is determined within the scope of the calibration measurement and then used as a constant for all the heart/time volume calculations.
It has been shown that the hemodynamic parameters determined by pulse contour analysis sometimes decrease in quality, as the duration of patient monitoring increases (calculated starting with the calculation measurement). In other words, the likelihood that the hemodynamic parameter obtained by pulse contour analysis will deviate from the actual physiological conditions by more than a certain predetermined measure increases.
In order to counteract this, the calibration measurement by thermo-dilution can be repeated at shorter intervals. However, this is connected with some effort and expenditure, particularly with the administration of a bolus injection, and this means additional stress for the patient being monitored, as well as putting a time burden on the personnel involved, thereby reducing the availability of that personnel for other tasks.